This invention relates to a method of fabricating an eraser and, more particularly, to a method of fabricating an eraser which contains low crystallizable polyolefin thermoplastic elastomer of a main basic material and normal synthetic rubbers having no hard segment portion for performing a crosslinking effect as required, i.e., any one or more of chlorinated polyethylene, ethylene-propylene terpolymer, ethylene ethyl acrylate, ethylene-vinylacetate copolymer, atactic polypropylene, styrene-butadiene rubber (SBR), nitrile rubber (NBR) and the like to not only provide superior erasing performance as compared with the conventional plastic eraser and to but also remarkably improve its properties such as its deterioration by aging, sunlight exposure, migration of plasticizer, weak body (low rigidity), weak tearing strength by an eraser retaining metallic band of the defects of the conventional eraser.
The conventional erasers sold at present in the market are largely classified into two types, one of which is a natural rubber eraser fabricated by mixing a factice and a filler with natural rubber of its basic material and vulcanizing the composition with sulfur in a semivulcanizing state, and the other of which is a synthetic rubber or plastic eraser fabricated by mixing a filler with emulsion polymerization type vinyl chloride (paste resin) of its basic material, kneading the composition with excess amount of plasticizer and then steam heating the composition to thereby gel the composition in semigelled state.
Although the natural rubber eraser incorporates advantageous properties such as capability of free selection of its rigidity, and large crack and tear strengths, it contrary has insufficient erasing capacity and undesirable using touch. This eraser also accomodates a difficulty in controlling its vulcanized state, namely, in case that the neutral rubber is insufficiently vulcanized, it becomes tacky, while in case that the natural rubber is excessively vulcanized, it loses its erasing properties to consequently occur irregular erasing capacity under quality control of the eraser and does not exhibit stably vulcanized state even after the eraser product is fabricated from the natural rubber, with the result that the eraser is adversely affected by exposure to sunlight and high temperature to thus progressively vulcanize the eraser, to thereby continuously increase its rigidity to cause the eraser to become brittle or remarkably hardened on the surface, and to thereby entirely loss its erasing function. Further, its excess unreacted sulfur blooms to the eraser surface to thereby disadvantageously lower the erasing capacity, and yellows the eraser to cause a great loss of its beauty appearance. Moreover, its production not only necessitates complicated technique requiring high skillfulness at the steps of kneading and vulcanizing crude rubber but also adversely affects its cost due to disposal of a great deal of chip rubber produced after the vulcanizing step without any utilization.
The synthetic rubber or plastic eraser containing mainly vinyl chloride, on the other hand, incorporates very preferable erasing capacity, favorite use feeling and outstanding erasing function better than the natural rubber eraser, however has a difficulty in employing excess amount of plasticizer and controlling the composition in semigelled state upon heating of the composition during production of the eraser and thus requires high technology to obtain uniform quality of eraser in the same manner as in the vulcanizing step of the natural rubber eraser. That is, in case that the plastic eraser is insufficiently gelled, it becomes brittle to cause the composition to be discretely collapsed and to become impossible to erase, while in case that the plastic eraser is excessively gelled, it is not worn in use to cause the eraser chips not to separate from the eraser and to thereby remarkably lower its erasing capacity. Although the plastic eraser product accommodates sufficiently higher resistances to exposure to sunlight and aging than the natural rubber eraser, it causes, since it is semigelled, its liberated plasticizer to bleed on the eraser surface to thereby migrate to other resin such as coated surface of a pencil or pencil case to erode the surface and to thus cause the surface to disadvantageously become tacky. The elimination of the disadvantages is expected, but they are not yet sufficiently countermeasured.
Although a pencil with an eraser or an eraser contained in a mechanical pencil is convenient in use, the eraser thus used requires not only general performance of the ordinal eraser but also, since it is engaged by a metallic band for retaining the eraser and is caulked thereby, its pinch-off or tear resistance by the burrs of the periphery and caulked portion of the band. The reason why the plastic eraser is not used for this application is that the plastic eraser does not have sufficient tear resistance and cannot endure this using pressure. Since the natural rubber eraser has almost durable tear resistance with certain rigidity and resistance to the using pressure, it is mostly used for the present pencil with the eraser and the eraser contained in the mechanical pencil. However, the natural rubber eraser still incorporates such disadvantages that has insufficient weather resistance and severe deterioration by aging as was described before, and particularly becomes impossible to erase in several days under exposure to sunlight. There have been proposed various trials to eliminate these disadvantages by improving vulcanizer or plasticizer or the like, however any of these trials has not yet been completed. There have also been proposed, as its plastic materials for the plastic eraser, employments of one or more of atactic polypropylene, ethylene vinylacetate copolymer (EVA), ethylene-propylene (EP) rubber, styrene-butadiene rubber (SBR), nitrile rubber (NBR), etc. as synthetic rubber and plastic having rubber-like properties in addition to polyvinyl chloride (PVC), however they do not have sufficiently satisfied high quality totally in view of its erasing capacity, mechanical strength and aging stability.
There has further been proposed a prior invention of an eraser having novel block copolymer in addition to natural rubber and polyvinyl chloride (PVC) as disclosed in Japanese Patent Publication No. 23450/1971, the rubber-like composition of which eraser is monovinyl-substituted aromatic hydrocarbon copolymer mainly having conjugated diolefin and styrene of basic materials, so-called styrene (SBS block copolymer) thermoplastic elastomer. When this composition is used as the eraser basic material, it is thermally unstable due to its unsaturated bond, and is difficult to continue kneading and molding steps at high shearing force and temperature for long time. The composition starts thermally deteriorating at higher than 150.degree. C. for long time to finally cause the composition to become brittle. The composition accommodates such disadvantages that, in case where its styrene content is increased in the eraser to improve its workability, it deteriorates its flexibility and exhibits residual strain particularly at high temperature to cause deformation of the eraser or increase in hardness of the eraser to thereby lack its wearing properties so as to undesirably deteriorate its erasing capacity.
Eraser stock material is, incorporating excessive wearing properties, not preferred. In this respect, of the present thermoplastic elastomers, urethane series incorporates the most plastic-like properties and high hardness, polyester, styrene and polyolefin series sequentially incorporate in this order increased rubber-like properties, and the polyolefin series accommodates the most suitable wearing properties as its basic material. The styrene series deteriorates its oil and solvent resistances, and when it is used for the eraser basic material, the resin itself solely incorporates insufficient erasing capacity. Although white factice is added, in order to increase its rigidity, to the resin or other basic material is blended therewith, it is difficult to mix a great deal of factice or basic material with the resin composition due to its deterioration of workability and to tendency of its embrittlement. When this eraser is used for an eraser with a pencil or contained in a mechanical pencil, it is readily torn by its metallic band to be thus insufficient in use.
There have also been proposed prior inventions of resin erasers which respectively employed ethylene-vinyl acetate copolymer (EVA) and atactic polypropylene (APP) as disclosed in Japanese Patent Publication Nos. 44245/1977 and 112429/1977. These resins are apparently rubber-like elastic material, however since they do not incorporate cross-linking structure, they tend to occur cold flow at cold working time to thus cause the eraser material to adhere onto paper surface and to thereby perform no erasing function but to contrary contaminate the paper surface. When the eraser is allowed to stand for under static pressure applied thereto, it is plastically deformed to cause the eraser to disable to retain its configuration. Furthermore, the eraser itself becomes brittle and cannot thus endure its tear by its metallic band.